The present invention relates to a rotary threaded fastener of the self-drilling and threading type, and more particularly to a fastener of that type which is manufactured by a pair of forging dies.
Various threaded fasteners of the self-drilling and thread forming type are known in the art, some of which are particularly adopted for manufacture by a milling process, while others are manufactured by a forging process which employs a pair of matched dies. A typical example of the type of drilling and thread forming fastener which is preferably formed by the forging process is shown in U.S. Pat. No. 3,395,603, issued Aug. 6, 1968 to E. J. Skierski and assigned to the assignee of the present invention. Fasteners manufactured in accordance with the teachings of this patent have met with wide acceptance as they provide a fastener which is simple to use, requiring no pre-drilling of the structure to which it is assembled, and fasteners of this type have proven to be easy to manufacture by mass production methods.
Recently, however, there has been a demand for self-drilling fasteners to be manufactured from high carbon steel, stainless steel, or metallic alloys which are of a hardness exceeding those of normal requirements. In some instances, it has been found that the forging dies employed to manufacture a drill point on these fasteners is susceptible to premature breakage, when these harder materials are employed. It is considered that the stresses inherent in present point forging die configurations trap the harder material of the headed blank being worked on during the point forging operation, and that the entrapment of the blank material and its subsequent cold flow generates excessive stresses in the point forging die, causing in some instances, a premature point forging die breakage.
With the wide use of the self-drilling and thread forming fasteners there has also been a requirement for the reduction in drilling time of the fastener. While existing point configurations which contain two flutes provide a satisfactory drill time in most applications, it is considered that they do not have sufficient clearance for chip flow to remove drilled material from the resultant hole, and thus to reduce drilling time. This occurs particularly when multi-layered assemblies of similar and dissimilar materials are operated upon . Such failure to remove drilled material can result in binding between the point and drilled hole surface causing excessive heat generation and resultant slower drill times.
The present invention therefore has as an object to provide a drilling and thread forming fastener of the type manufactured by a die forging process, which is effective to facilitate an increase in point forging die life.
Another object of the present invention is to provide a drilling and thread forming fastener wherein a greater clearance for escape of drilling chips is provided in the resultant drilled hole, during the drilling process.
Another object of the present invention is to provide a drilling and thread forming fastener which is susceptible to a reduction in drilling time over those drilling and thread forming fasteners of the prior art.